Electrostatic transducers have been produced for some time. These transducers generally involved a metallized plastic membrane stretched across a flat or curved surface of a conductive backplate. A gap between the membrane and the backplate is maintained to create a controlled capacitance. Tension on the membrane maintains the gap; however, humidity and other ambient conditions cause the membrane to elongate which, in turn, cause substantial problems in transmitting and/or receiving pressure energy waves. Some of these early transducers use metal membranes and depend totally upon the spacing of the membrane from the backplate for the capacitance needed to transmit waves by vibrating the membrane and receive echoes by the membrane being vibrated. Such prior systems with fixed gaps were not used extensively for detecting the position of objects due to low sensitivity, high voltage leakage and other problems. To correct some of the difficulties of these early electrostatic transducers, an electret type transducer was developed wherein the membrane was a plastic film covered by a metal, such as vacuum deposited gold. A chemical charge was applied to the film, which was held spaced from the major flat surface of the backplate. Such transducer was relatively inexpensive; however, the chemically created electric charge was relatively small so that the output of the electret transducer was and is very small. Further, the charge on the film dissipated, or was reduced, by atmospheric conditions. All of these difficulties caused electret transducers to be useful for only limited purposes. They were not adapted for the rigors of industrial applications requiring high precision location of objects. These prior electrostatic transducers required a relatively wide gap, which resulted in poor sensitivity and substantial difficulty in controlling response; therefore, such transducers could not be employed for controlling robotics or in other environments requiring repeated and accurate determination of the spatial relationship of an object from the transducer or a probe housing the transducer.
Capacitance type electrostatic transducers with all their disadvantages and limitations, both in repeatability and in production, were still considered to be the least expensive type of transducers for the purposes of sending and receiving ultrasonic waves for range finders. To rectify the many problems in this type of device, it was proposed to produce a plurality of grooves or other striations in the major surface of the backplate to provide several protrusions extending from the surface. These protrusions contact the plastic film portion of the membrane. In this manner, the limited areas of contact by the protrusions created several intermediate cavities that acted as a capacitor. The spacing under the membrane could be reduced substantially over prior transducer designs requiring a fixed gap between the backplate and membrane with no actual contact. This smaller spacing increased sensitivity.
With a biasing voltage applied across the metal surface of the membrane and the surface of the backplate, a burst of high frequency voltage across these members causes vibration of the membrane and thus a high frequency, or ultrasonic, wave to be transmitted from the transducer. By maintaining the biasing voltage, echoes received by the transducer from various objects could vibrate the membrane causing high frequency voltage fluctuations across the metal layer and backplate. In this manner, ultrasonic signals could be transmitted and the echoes could be received by transducers wherein the backplate had protrusions engaging the plastic film of the stretched membrane. This concept was a substantial improvement over prior electrostatic transducers for use in transmitting and receiving ultrasonic waves; however, there were substantial limitations as set forth in U.S. Pat. No. 4,081,626. The basic disadvantage of these transducers using grooves or striations to create the capacitance between points of contact with the membrane is caused by leakage voltage from the backplate to the plastic or insulating layer of the membrane. For this reason, the membrane becomes charged rapidly and biasing voltage appears across the membrane. Consequently, no voltage appears across the gaps created between the striations. The output of such a transducer decreases somewhat exponentially. This leakage current flow can also cause a breakdown of the film so that the membrane could be punctured. One way to solve this basic problem with prior transducers was provision of a power source having an increased available current. The leakage current was provided by more available current. Such high current operation was not desirable; therefore, the problem of charging the insulation portion of the membrane was reduced by providing less contact area between the backplate and membrane. This concept is discussed in U.S. Pat. No. 4,081,626 wherein the striations or grooves are provided with further surface roughening features to reduce contact area. The concept of reducing contact area is also taught in U.S. Pat. No. 4,246,449. In this patent, the protrusions are machined by the well known process of electrical discharge machining (EDM). Sand blasting, more grooving or EDM were all suggested as processing procedures for reducing the contact area between the backplate and the membrane; however, such manufacturing processes were not controllable and did not produce the desired sensitivity or consistency needed for electrostatic transducers to be used as a device for determining distances. Surface treatment of the ridges used to create capacitor cavities in the backplate would vary from transducer-to-transducer. This problem is recognized and attempted to be solved in U.S. Pat. No. 4,311,881.
The efforts to reduce the area of surface contact on ridges between the grooves on the backplate still allow substantial leakage current flow; therefore, the biasing voltage ultimately appeared across the membrane to reduce the effective time during which the transducer can be used. In view of this limitation, a burst of energy causing an ultrasonic wave had to be stopped rapidly so that an echo could be received without distortion from the outgoing ultrasonic wave. There was just not sufficient time to allow transmission of a wave and reception of an echo before the membrane was overly charged by leakage current. For that reason, the range of the transducer was substantially limited. Roughing of the backplate surface is not now and was not a solution to the basic leakage problem created by grooved type transducers, even though these transducers did correct deficiencies of earlier electrostatic transducers. The use of a grooved surface engaging the film of a membrane reduced the capacitor gap, but, caused current leakage problems and possible blow through or shorting of the membrane. These disadvantages of various transducer designs were somewhat counteracting and caused electrostatic transducers to be designed without actual total correction of any problem.
As so far described, transducers for the purpose of range finding have employed ultrasonic technology wherein the membrane is vibrated at a given frequency to create an ultrasonic pressure wave that progresses toward and is reflected from an object. These devices have not been operated by shock waves, as used with piezoelectric crystals in prior U.S. Pat. Nos. 4,326,115 and 4,459,526. Shock waves are more distinct and can be detected better than high frequency ultrasonic waves dependent upon vibration of the membrane of the transducer.